1. Field of the Invention
This invention relates to the field of operational amplifier circuits and, more particularly, to operational amplifier circuits having minimal cross-over distortion while receiving rail-to-rail input voltage swings and providing rail-to-rail output voltage swings. In addition, the present operational amplifier circuit operates at a power supply voltage as low as two volts when fabricated in modern CMOS fabrication processes.
2. Description of the Relevant Art
Operational amplifier (op-amp) circuits have many important applications in electronic circuits, particularly those circuits which utilize feedback to produce their operation. Among the circuits which may be built with op-amps are the unity gain amplifier which conveys an output voltage substantially equal to its input voltage. Another exemplary circuit is an integrator in which the output voltage is the integral of the voltage difference between the input terminals over time. Numerous other examples of the use of op-amps are familiar to those skilled in the art.
An op-amp circuit is an amplifier circuit having an output voltage which is proportional to the voltage difference between two input terminals. The operational amplifier circuit is popular because it is an excellent voltage amplifier. For example, op-amps are characterized by extremely high input impedances and extremely low output impedances, both of which are desirable in a voltage amplifier. Since the input impedance is high, the current drawn by input terminals to the op-amp is low. Therefore, the source of the input voltage need not be capable of a large current drive. Because the output impedance is low, the voltage provided at the output of the op-amp is relatively stable for large current flows at the output.
Op-amp circuits are often configured with NMOS and PMOS transistors. As will be appreciated by those skilled in the art, NMOS and PMOS transistors are two types of transistors that are formed in a complimentary metal-oxide-semiconductor (CMOS) fabrication process. PMOS transistors and NMOS transistors have four terminals (or connection points): a gate terminal, a source terminal, a drain terminal, and a bulk terminal. Electric current flows from the source terminal to the drain terminal of a transistor when a voltage applied to the gate terminal has either a higher or lower value then the voltage applied to the source terminal, depending on the transistor type. A PMOS transistor is a transistor in which current flows if the voltage applied to the gate terminal is lower than the voltage applied to the source terminal. An NMOS transistor is a transistor in which current flows if the voltage applied to the gate terminal is higher than the voltage applied to the source terminal. The bulk terminal is connected either to the source terminal of the transistor or to a proper bias voltage.
In both the PMOS transistor and NMOS transistor, the difference in voltage between the gate terminal and the source terminal must be larger in absolute value than a certain voltage before current flow begins. This certain voltage is referred to as a "threshold" voltage and is the voltage required to form an energized channel between the source and the drain diffusion regions in the PMOS transistor or NMOS transistor. As will be appreciated by those skilled in the art, a transistor is formed on a substrate by diffusing impurities into two regions (a drain diffusion region and a source diffusion region). The two regions are separated by a distance of undiffused substrate material called a channel, over which the gate terminal is constructed. By applying a voltage to the gate terminal of the transistor, the channel is energized such that current may flow between the source diffusion region and the drain diffusion region.
Theoretical op-amp circuits are capable of "rail-to-rail" input and output operation and perfect linearity. The "rails" of an op-amp are its power supply voltages. An output is said to transition from rail-to-rail if the output conveys a voltage substantially equal to one of its power supply voltages in response to a particular input voltage, and the output conveys a voltage substantially equal to the other one of its power supply voltages in response to another input voltage. An amplifier circuit is linear if the output voltage is a constant proportion of the input voltage for any input voltage within the operable voltage range of the amplifier. That is, if the output voltage is plotted as a function of the input voltage, the result is a straight line. Unfortunately, conventional op-amp circuits (including CMOS op-amp circuits) have not been capable of rail-to-rail operation. Additionally, these op-amp circuits have not been capable of linearity. A particular problem with linearity in many op-amps is "cross-over distortion". Cross-over distortion is a disruption of the linearity of the output voltage when the input voltage is near a particular voltage (referred to as a "cross-over voltage"). For input voltages near the cross-over voltage, the output voltage remains fixed instead of assuming a value which is a constant proportion of the input voltage. As the input voltage moves farther from the cross-over voltage, the output voltage resumes its linear rise or fall. An op-amp circuit having low cross-over distortion and rail-to-rail output voltage capability is desired.